The Effects of Pouring Concrete in Cold Weather

concrete pouring temperature graphic
First, let’s define cold weather with respect to pouring concrete. Any time you have three consecutive days where the average daily temperature is less than 40°F, or if the temperature is lower than 50°F for less than half of any of the three days–that is undesirably cold for concrete. Pouring concrete in cold weather will have a detrimental affect on concrete curing for several reasons.

How Does Concrete Cure?

Concrete transforms from a liquid to a solid material through a chemical reaction. The speed of the reaction depends upon the temperature of the concrete. When the weather is warm, the reaction proceeds quickly. When it’s cold and the ground hasn’t been thawed, the reaction slows down. That’s the problem: the concrete needs to harden as rapidly as possible to resist pressures caused by water freezing within the concrete.

Slower Chemical Reactions

If the temperature is too cold, the concrete may not have reached a minimum strength of 500 psi soon enough to resist the effects of freezing temperatures. If your concrete isn’t protected with concrete curing blankets after it’s poured, it may cool too rapidly, slowing the chemical reaction.

Poor finishing techniques can also doom your slabs. Freshly poured concrete often bleeds. The water in the mix floats to the top, since it’s the lightest ingredient. Floating or troweling this water into the concrete weakens the top layer. Troweling the concrete too early can seal this bleed water just below the surface as well. If your slab is then exposed to freezing temperatures several days later, this water can freeze and fracture the top layer. Using a concrete curing blanket can eliminate the potential of freezing.

Read how Powerblanket solved concrete issues at the Statue of Liberty.

Concrete Can Be Successfully Poured in Cold Weatherpouring a concrete foundation at a jobsite

How to avoid a bad concrete pour:

  • Never pour concrete on frozen ground, snow, or ice. 
  • Be sure to order air-entrained concrete. Request a heated mix or order 100 lbs of extra cement for each cubic yard of concrete. This extra cement helps develop early strength.
  • Be sure the concrete is ordered with a low slump (drier mix). This minimizes bleed water.
  • After the final finish is completed, cover the concrete with a concrete curing blanket. The heated concrete blanket will prevent freezing and keep the concrete at an optimal curing temperature.
  • After about three days, remove heated concrete blankets to allow the concrete to air dry.

Powerblanket Concrete Curing Blankets

If you use Powerblanket Concrete Curing Blankets to pour in cold weather, your cement will cure 2.8 times faster than with conventional insulated blankets. Time and convenience are critical factors when planning out a project in the winter, and Powerblanket has you covered. Our goal is to provide solutions to problems, and give you total temperature control over every process and aspect of your business.

We Solve Problems

From drum and barrel heaters to pipe wraps and snow melting mats, Powerblanket is dedicated to helping your business grow, saving jobs, and improving your bottom line. Whether you need to extend the pouring season, or you are trying to survive an early winter, know that you have total temperature control with Powerblanket.

 

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Curing Cold Concrete: Electric vs. Insulated Blankets

Electric concrete blankets versus insulated blankets. Which one is best and why is it Powerblanket?

Not All Concrete Insulated Blankets Are Created Equal

If you happen to read almost any do-it-yourself guide, you will likely see a sentence that says, “If temperatures are below 40°F, use insulated blankets or straw to keep your freshly-poured concrete from freezing.” Well, that sounds nice and easy, but simply keeping concrete from freezing isn’t enough.

Why Heat Concrete?

The optimum range for proper concrete curing is between 65° – 85° F. When concrete dips below that range, ice crystals can form, cure strength will be compromised, and several temperature based defects can occur. Concrete pouring is a critical stage in any construction project and problems derail progress when it needs to be repaired or re-poured. Solely insulating concrete during curing seems appealing and money-saving, but can lead to costly time delays and expensive re-works.

What is Thermal Insulation?

Simply put, thermal insulation reduces the transfer of heat between objects. There are a myriad of both natural and synthetic materials used to insulate objects in different industries. When thermal insulation is placed on something that is already warm, it will prevent heat from escaping.

The chemical reaction of concrete formation is exothermic and does create some heat. This heat can be trapped (to some degree) with the use of insulating blankets or straw. But if it’s cold enough outside, insulating blankets won’t keep the temperature at an ideal level for maximum curing.

Insulation + Heating

When pouring concrete in cold weather, providing thermal insulation is only half the battle. Most insulated blankets will not provide enough surface contact to effectively insulate your concrete on really cold days. Combining the protective power of insulation plus even heat distribution gives you an advantage over mother nature.

Electric concrete heating blankets provide the protective layer of insulation and an extra boost of heating power to keep your concrete between 65° and 85° F. Concrete blankets have 10x more surface contact than other blankets. They efficiently transfer heat down into your concrete while heavy-duty weather-proof insulation protects it from the elements and heat-loss.

Thaw Before Your Pour

No insulating blanket on its own will thaw frozen ground, and pouring concrete on frozen ground can have serious repercussions.

electric ground thawing blankets

Electric ground thawing blankets are the best way to ensure your concrete pour gets off on the right foot. Using an electric ground thawing blanket prepares the ground before you pour so ambient temperatures above and below your concrete are at ideal levels.

Concrete Curing BlanketsMany factors have an effect on concrete cure time, such as the type of mix used, the size and shape of the slab, and the ambient weather. Of all these factors, weather and temperature is the most unpredictable. The colder the temperature, the longer concrete will take to set and reach its proper strength. During cold weather, a conventional insulated blanket will cause the concrete to take longer to cure and will result in a weaker concrete.

For example, concrete kept at 70°F will set in approximately 6 hours, whereas concrete kept at 40°F will take 14 hours to set. Concrete maintained at 70° F will reach a compression strength of 2,700 psi in 3 days; concrete maintained at 40° F will only reach a strength of 1,200 psi in 3 days.

This disparity in strengths makes a huge difference in whether your construction project can proceed. If concrete takes too long to set, cure, and strengthen, your project can be significantly delayed. Some concrete manufacturers find it too problematic to operate during the winter, creating a costly off season.

Concrete Solutions From Powerblanket

With Powerblanket, there is no off season. The difference between conventional insulating concrete blankets and electric concrete curing blankets is clear, and our electric blankets are a cut above the rest. By combining heavy weatherproof insulation with electric heating components, Powerblanket can cure concrete 2.8 x faster than conventional insulated blankets.

Our ground thawing blankets melt ice and snow and prepare the ground before your concrete is even poured. Our concrete curing blankets then maintain the temperatures you need to more quickly cure your slab to a strong finish. Powerblanket can eliminate costly weather delays, ensure your project proceeds on schedule and that your concrete will be long-lasting and durable. Contact us to find the right concrete solution for your needs 855.440.0208 or [email protected]

Learn More About Concrete Curing Blankets

Massman Construction Co. teams up with Powerblanket on the Christopher S. Bond Bridge

Christopher S. Bond Bridge in Kansas City Missouri

As motorists travel their daily commute, they appreciate infrastructure upgrades such a freshly paved roads, smoother drives, and improved routes. However, they often don’t realize the skill, effort, and creative problem solving that goes into the construction itself. 

In April 2008, Kansas City embarked upon a $245 million project that included the renovation of more than 4 miles of Interstates 29 and 35, and the construction of the Christopher S. Bond Bridge. The Bond bridge would replace the Paseo Bridge, which had spanned the Missouri River since 1954. The Missouri Department of Transportation (MoDOT) estimated that the project would require 7,000 tons of steel and 50,000 cubic yards of concrete. 3 contractors (Clarkson Construction Co., Massman Construction Co., and Kiewit Construction) formed a joint venture partnership known as Paseo Corridor Constructors (PCC) to bid on the project. The PCC partnership won the contract, and construction began in April 2008. 

Bridging The Gap

In late 2009, Massman Construction, a century-old company with a reputation as one of the country’s most successful bridge builders, encountered a unique challenge during a critical portion of the new bridge. The bridge deck would be comprised of precast concrete panels, each weighing about 60,000 pounds. These panels were formed in a precast yard offsite and then transported to the bridge. Once the panels were placed, Massman was left with an 18 inch wide, 12 inch deep void that would need to be filled in with poured concrete in order to solidify and connect the bridge deck. Unfortunately, to stay on schedule, this concrete pour would have to be accomplished in the dead of a mid-west winter.

Dale Helmig, Massman project manager, said that in order for the concrete to cure properly, “we had to maintain our concrete pour at a minimum temperature of 46° F for 7 consecutive days. We were on a tight schedule and looking at outdoor temperatures at or below 0° F, we had to find a way to keep the concrete warm.” Massman contemplated concrete heating methods and realized heating the top layer of concrete was not the problem. They needed a way to protect the bottom of the fresh concrete from constant exposure to the winter elements below the bridge.

The crew decided the only way to keep the bottom layer of concrete warm, was to completely enclose it. A river barge would float underneath the bridge, hoist a plywood enclosure, and force heated air at the bottom of the bridge. This solution would be excessively expensive, time-consuming, and there was no guarantee radiant heating would be efficient enough to work.

A Powerful Solution

Fortunately, Massman connected with a local equipment dealer that had an alternative solution; electric-powered concrete curing blankets from Powerblanket. Unlike the radiant heat from the barge proposal, Powerblanket products use conduction to deliver direct, even heat. 

Powerblanket diagram on conduction for concrete curing

These electric concrete blankets help cure within a desired temperature range. This seals in the concrete’s hydration reaction, and protects it from thermal cracking and surface defects.

“We gave Powerblanket’s engineers the boundary conditions we had to keep the concrete pour within — at least 46° F at the bottom but no warmer than 82° F at the top,” said Helmig. “They performed the calculations and determined the appropriate blankets needed to get the job done.” In order to heat the entire 12 inch deep slab, Powerblanket recommended our EH0612 blankets to counteract the extremely cold conditions beneath the bridge deck. For all 34 heating blankets, Powerblanket also created custom external digital thermostats so that Massman could constantly monitor their temperatures. 

Pouring fresh concrete in direct contact with the freezing cold precast panels also presented a challenge. Rapid temperature change in concrete will cause it to thermally crack. To protect the structural integrity of the bridge, Powerblanket provided 6 foot x 12 foot concrete curing blankets that easily exceeded the 18 inch trough. The additional blanket width pre-heated the precast concrete panels, making it safe to pour the fresh concrete.

concrete

A Strong Finish

Massman began the week-long concrete cure, during which the outdoor air temperature hit a low of -5° F. Crew members used probes to take temperature readings several times a day. “It was awfully cold, but the blankets did their job keeping the concrete temperatures within spec,” said Helmig. “We achieved the PSI levels we had targeted, and it turned out to be a successful operation. We didn’t have the time to wait out the winter for the temperatures to become more moderate. The Powerblanket products certainly played an important role in keeping us on schedule and not having to postpone the pour.”

Helmig has since moved on to another ongoing Massman project in St. Louis, and says he may be using the blankets again there. “We were committed to doing something to stay on track,” said Helmig. “Using Powerblanket products turned out to be the fastest, simplest means of getting the job done correctly.” Work progressed so well that by late October 2010, the Christopher S. Bond Bridge had already opened to traffic crossing the Missouri River.

Thanks to partnership and ingenuity, 102,000 daily motorists can rely upon the Paseo Corridor and Christopher S. Bond Bridge. Powerblanket is proud of our part in the construction of a strong and stunning work of American infrastructure. 

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